1. Field of the Invention
The present invention relates to a disc-shaped, high storage density information carrier that is optically readable from one or from both sides in reflection or in transmitted light, whereby the information exhibiting a microstructure is disposed in a concentric, annular surface of the information carrier disc.
2. Description of the Prior Art
Information carrier discs of the type set forth above are known, in general, for example from the German published application No. 28 39 359 which corresponds to the British Pat. No. 2,004,404, both of which are fully incorporated herein by this reference. The microstructure representing the information is impressed into the disc surface on one side of the disc, is provided with a reflective metal layer and covered with a protective layer. The disc is not read at the side having a protective layer, but from the opposite side, this presuming a transparent material for the carrier material in this case. A laser beam is focused in the plane of the information structure so that dust particles and contaminants on the surface of the read-out side of the information disc have a far less disruptive effect and would be the case if reading of the information were to occur on the side of the protective layer covering the information structure.
As the above-mentioned reference also discloses, it is possible to read such an information carrier reflectively from both sides given the combination of two carrier discs carried on the sides of the information structure forming a sandwich disc structure. In addition to such disc-shaped information carriers, optically readable through reflection, embodiments are also known in which the reading occurs by light transmitted therethrough.
As practice has shown, it is not only imperative for a faultless reading of such disc to keep the disc surface on the read-out side as free as possible of dust and other contaminants, rather it is also important that the discs remain scratch-free on the read-out side when manipulated. It is precisely scratches, in particular, that result in the consequence that the light of the laser beam is diffusely reflected in the region of a scratch and, therefore, the reading can be considerably disrupted due to a lack of faultless focusing of the laser beam in the plane of the information structure. In addition, such scratches cannot be subsequently removed.
It is already known from the Japanese published application No. 58-48089, fully incorporated herein by this reference, to provide single-sided, optically-readable, disc-shaped information carriers having an elevated protective edge on the read-out side so that only the protective edge can slide on a support when the disc is deposited on the support.